Electrical circuits



Maylz, 1942. l A. w. BARBER 2,282,696

y ELECTRICAL aucuns Filed Nov. 3, 1938 2 sheets-sheet 1 1. ,4 n ,002 INVEVNTOR loa, d 539cm@ .ar el o e w -gam/w 0'2 0' "o ATTORNEY May 12 1942- A. w. BARBER 2,2825696 ELECTRICAL CIRCUITS Filed Nov. 3. 1938 2 Sheets-Sheet 2 2MB Y A E 057.' AMI? f/VERTOR lm/ABLE FREQufA/c OSC/L L TOR INVENTOR grad Wa/rte -ATTORN EY Patented May l2, 1942 Y g UNITED [STATES PAT ELECTRICAL CIRCUITS Alfred W..Barber,' Flushing, N. Y., assignor to Radio Corporation ofK America, a corporation of Delaware Application November s, 193s, serial No. 238,538

` the oscillator' inductance are changedV simul- 1 Claim.

The present invention relates to electrical oscillator systems and in`particular'to systems embodying frequency modulation'of a thermionic vacuum tube oscillator.

One object of the present invention is to provide a frequency modulator in whichthe mean '5T' frequency may be varied over a range of frequencies while maintaining a substantially constant width of frequency modulation excursion;

Another object isto embody such a constant band frequency modulator in aA generator of frequency'modulated signalsV useful in various applications.

Still another object is to improve and simplify automatic tuning systems as applied to radio receivers by application of my present constant band-width frequency modulation system to the tuning correction system of such receivers.

In my U. S. Patent Number 2,082,317 issued on June l, 1937, is shown a method of producing and if controlling a frequency modulated signal. 'A thermionic vacuum tube oscillator `is partially tuned by the input capacity of a second tube having a. condenser connected between grid and plate and a resistor plate load.L The input capacity is varied by'varying the gain' of the ca-A pacity tube and the oscillator is hence frequency modulated. A cathode ray tube is used* to indicate the response of a circuit to the frequency taneously, an all Wave signal'generator may be produced lhaving substantially constant frequency modulation band Widths over an extended frequency range. f

The present invention maybe more fully *un-A derstood from the detailed description lof the various figures of the drawing.` f

In the drawings:

Fig. 1' shows a simpliedcircuit embodying the -J present invention.

Fig. 2fshoWs a modified simplied circuit.

Fig. 3` shows a complete circuit diagram ofl frequency determining inductance to set Vthe mean frequency value of the oscillations.

' pacity producing tube 4 -having la cathode 'I-,`grid modulated signal against the signal causing the frequency modulation which may conveniently be'taken from the local power line. Itis often desirable to vary the mean'frequency of the frequency modulated signal over a range of values. across the Voscillator inductance; the width of the frequency modulation bandwill become continuously smaller as the mean frequency is decreased.

The present invention embodies means for producing a substantially constant frequency modulation band width as the mean frequency is varied. This equal band width condition is obtained by varying the capacity between plate and grid of the capacity tube in order to'vary If this is done by varying a condenser and the tube characteristics.

the mean frequency and by adjusting the capacity across the oscillator inductance to a suitable value. With no capacity across the oscillator inductance except that produced by the capacity tube, the band width of frequency modulation will increase proportional to the mean frequency. However, an added capacity in shunt with the oscillator inductance tends to decrease the frequency modulation band width and by an amount which increases with the mean frequency.

soV

6 and plate 5 is connected with its grid to cathode circuit across oscillator inductance v2, thru blocking Condenser 8l and `cathoderbias resistor I0 by-passed by `condenser II. Plate 5 is connected -to a variable load resistor 9 'anda vplate energizing battery I6. Heater energizing means is not shown but will be understood to be any.,

conventional means. A variable condenser I4 is connected between grid 6 and plate 5 and grid 6 receives a signal from source I3V thru grid-resistor I2. The effective capacity, added by the capacity producing tube 4, across inductance 2 Will be capacity I4 times one plus thegain of tube 4 as determined by resistor 9, voltage I3 The frequency of source I may hence be varied by varying voltage I3. If source I3 is a yperiodic voltage, the capacity supplied by tube 4 and hence the frequency of source I will `be periodically varied in accordance with this voltage. The magnitude of this' periodical frequency variation will depend on the various constants as stated above and may be termed the frequency modulation band width. If condenser I4.onl y is associated inthe frequency determining circuits of'tube 4 andv source I, the band width of frequency modulation Will increase as the mean frequency is increased by decreasing capacity I4 as shown in curve a of Fig. 5. Mathematically, the band width of frequency modulation Af is equal to the mean frequency F times the capacity tube gain A divided by 2 as follows:

- i aztec ENICFFICE y;

is decreased at the higher mean frequencies as shown in curves b and c in Fig. 5. This is due to the fact that as capacity I4 is decreased, a

smaller and smaller part of the total frequency determining capacity is modulated by the action of tube 4 and voltage I3. Curves a,-b and c are for band widths. against mean frequency where capacity farads at mean frequency of 1.0 to 500 micromicrofarads at minimum mean frequency. Curve a is for capacity Sequal to zero, curve b is for capacity 3 equal toV 10 micro-microfarads, and curve c is for capacity 3 equal to 50 micromicrofarads. Linesy e and f have been drawn on Fig. indicating a two to one frequency range. It will be noted that within this range the total departure from constancy of band width for curve a is 100 percent, for curve b is 64 per cent and for curve c is only A21 per cent. Lines d and .y have been drawn covering a three to one frequency range showing that curve a departs 200 per cent from constancy, curve b departs 105 per cent and curve c departs 50 per cent; Since it will generally be desired to use only these indicated portions of the curves, the maximum frequency may be limited to points f or g by a minimum capacity control condenser I5 in shunt with main condenser I4. It will be understood that a part orall of either or both of condensers may be supplied by stray circuit capacities and the minimum capacity of condenser I4 if these capacities are of the proper magnitudes for the purpose set forth above.

Fig. 2 is similar to Fig. l except that the power factor of the input capacity of tube 4 is improved by coupling a load resistor I9 to plate 5 thru transformer I1-I8. Forxed fre- I4 is varied from 10 micro-micro# quency applications, this transformer may be tuned to the frequency of source I by means ofcondenser 2U. The secondary I8 should-have fewer turns thanV primary I1 so that a larger amplified Voltage per ohm of the load resistor I9 is produced than if resistor I9 were directly connected to plate 5. In Fig. 2 the condensers 3 and I5 of Fig. 1 are shown omitted being replaced by the stray capacities 2I and 22 respectively.

Fig. 3 shows a complete circuit diagram of a signal generator embodying one form of the present invention and including a cathode ray tube for observing response characteristics of apparatus subjected to the frequency modulated signal. The capacity tube 4 includes a cathode 1, control grid 6, two further grids 15 and 16 and plate 5. Grids 15 and 16 are connected to plate 5 so that the tube operates as a triode although other connections may be used such as tetrode or pentode circuits. Cathode 1 receives its bias from cathode current flowing in resistor IllA bypassed by condenser II connected to ground G. Frequency control condenser I4 is connected between grid 6 and plate 5 in order to provide constant frequency modulation band width as set forth above. Plate load resistor 9 is provided to produce the desiredvgain for frequency modulation. Resistor 9 maybe varied by means of switch 14 in order to adjust the frequency modulation band for different frequency bands of the generator. The oscillation generator comprises the vacuum tube 24 having cathode 26, control grid 25 and plate 23 coupled to one of the oscillator inductances 2, 2 or 2 by means of range change switches 3I and 32. Control grid 251 receives bias from grid leak 21 and grid condenser 28`. Plate voltage is supplied thru resistor 29 .and the oscillator inductances are coupled thru blocking condenser 30.

Output from the oscillator is supplied to Apparatus under test thru lead Y35 from a variable tap on output potentiometer 34 coupled to the oscillator inductance -being used to generate oscillations thru coupling condenser 33. Capacity tube 4 controls the oscillator frequency by coupling grid 6 to the oscillator .inductances thru blocking condenser 8. Frequency modulation under control of the'power line frequency is provided bydriving grid 6 from a tap on a potentiometer 58 supplied from winding 60 on the power transformer 6I thru the radio frequency isolating Achoke 51. The system so far described supplies a substantially constant band width frequencymodulated signal by Varying condenser I4 with continuous control of mean frequency and operableover a number of frequency bands byk simultaneously switching oscillator inductances and capacity tube load resistor by means of switches 3l, 32 and 14 to Apparatus under test.

Powerfor operating the system is supplied from a suitable power supply such as that shown. Power transformer 6I includes a primary winding 62 connectable to a power line plug 63 thru on-off switch 64; heater winding 65 for supplying rectifier 66; high voltage winding 61; and frequency modulation control voltage winding 60. The power supply also includes filter chokes 68 and 69, filter condensers 10, 1I and 12, and bleeder resistor 13. A cathode ray tube 31 is provided for observing the response of the Apparatus under test to the frequency modulated signal against the driving signal of power line frequency. Cathode ray tube 31 includes static deflection plates 38, 39, 40 and 50, cathode 54, control grid 53, focus anode 52, final anode 5I and cathode heating means not shown. Suitable electrode ypotentials for tube 31 are supplied from points on voltage divider 56 connected across high voltage battery 11 or other suitable high direct current voltage source. A vertical deflection is obtained indicating the response of the Apparatus under test to the frequency modulated signal by connecting the apparatus output to vertical defiecting plate 38. A horizontal axis is provided by connecting horizontal deflecting plate 40 to the power line voltage from winding 66 thru resistor 59 and across condenser 55. Condenser 55 and resistor 59 are adjusted to vary the phase of the horizontal deflection providing any 'desired relation of the horizontal deflection and generator frequency.'

An additional application of my present invention is shown in Fig. 4 where my constant band width modulator is applied to automatic tuning in a radio receiver. An automatic tuning system for superheterodyne receivers has been shown by S. W. Seeley in U. S. Patent No. 2,121,103, issued June 21, 1938, in which thel intermediate frequency signals traversing the intermediate frequency amplifier are applied to a discriminator circuit which is in effect a pair of tuned coupled circuits feeding'a double diode detector. The phase of the rectified voltage produced by the discriminator changes sign when the intermediate frequency signal mean frequency equals the resonant frequency of the'tuned coupled circuits. A frequency correcting device may be operated from this rectied voltage tuning or correcting the variab-le oscillator frequency of the superheterodyne so that the intermediate frequency signal is centered on the discriminator mean frequency.

My present constant band system has a great advantage when applied to such automatic tuning systems since it will give equal correction throughout the tuning range of the oscillator. The modulating voltage of the frequency modulation system is simply replaced by the rectified output of the discriminator and the capacity tube is connected across the inductance of the variable frequency oscillator. Fig. 4 shows a superheterodyne radio receiver comprising a radio frequency amplifier 'I9 connected to antenna A thru coupling condenser 'I8 and to ground G, rst detector 80, intermediate frequency amplifier 8|, second detector 82, audio frequency amplifier 83, speaker S4, variable frequency oscillator 86, and discriminator 85 labeled control voltage generator. The control voltage generator produces a bias Voltage which depends on the mean frequency of the intermediate frequency signals as described above. This bias voltage is applied to grid 99 of the capacity control tube v98 by means of grid resistor 91 and across filter condenser |06 which removes radio frequency voltages from the lead to and from control unit 85. Capacity control tube 98 has a cathode |0|, a plate and is connected with a cathode bias resistor |02 by-passed by condenser |03, plate load resistor |04 which may be varied by means of tap switch |01, a source of plate voltage +B, not shown, by-passed by condenser |05 and a heater for cathode |0| also not shown. In order to control the grid to ground capacity of tube 98 in accordance with the present invention, the variable oscillator tuning condenser 95 and its series padding condenser 94 are connected in series between plate |00 and grid 99, and grid 99 is coupled to the variable frequency oscillator inductance 81 by means of blocking condenser 93. As long as the bias supplied from control generator 85 is zero, the circuit operates in the usual manner in which condenser 92 is varied in tuning the receiver causing oscillator 86 to cover a desired frequency range. Capacities 94 and 95 are made slightly smaller than they would be without the capacity tube 98 since their capacity is multiplied by a small factor by the tube gain. When a station is tuned in, if the resulting mean intermediate frequency is higher than the desired intermediate frequency, assuming the oscillator frequency of 86 is greaterV than the received signal frequency, control generator 85 will generate a positive bias increasing the gain of tube 98 and the capacity produced by this tube and hence reducing the oscillator frequency until the mean intermediate frequency is corrected. If the mean intermediate frequency is too low, the generated bias is negative reducin-g the `capacity supplied by tube 98 and raising the oscillator frequency until the mean intermediate frequency is corrected. It is important in an automatic tuning system of this type that the correction of the oscillator frequency be limited as otherwise the system would tune into undesired channels. If the correction inthe usual capacity control system is made correct at one of the frequency range, it will be too small or too great at the other end. This present system will give substantially constant `correction at any position of condenser 92 and hence equal corrections at both ends of any frequency band. Condensers 94 and 96 supply in conjunction with the various stray capacities of the system the desired shunt capacities to cause the system to produce equal increases and decreases in capacity for the same control bias changes regardless of the setting of condenser 92. If the stray capacities are of the correct value, either or both of condensers 94 and 96 may be omitted. In order to adjust the correction effect of tube 98 as bands yare switched in an all wave receiver switch |01 is provided to change the load resistor |04 in accordance with the wave change switch. In general, resistor |04 will be decreased as the frequency of the oscillator is increased. Switch |01 may be ganged with wave change switches changing carrier circuit inductances and oscillator inductance 81. Oscillator tuning condenser 92 may be ganged with carrier circuit tuningcondensers in the usual manner. Pad-ding condenser 94 is for thepurpose of decreasing the oscillator frequency range and causing it to track at a fixed frequency difference with the carrier tuned circuits in the usual manner.

While a preferred form of my present invention has been shown and two applications, it is not intended to limit the modifications which may be made except as set forth in the appended claim.

Y What I claim is:

A variable frequency oscillator system comprising a rst thermionic tube having a cathode grid, and plate, a plurality of three-terminal coils, means to connect one terminal of each coil together, said connecting means being grounded, a parallelly-connected condenser and resistor connected between said grid and said connecting means, a rst switch for selectively connecting a second terminal of each of said plurality of coils to said plate, a second switch for selectively connecting a third terminal of each of said pluraliti7 of coils to said cathode, a condenser connected between said first switch and said second switch,

a variable output circuit comprising a serially connected resistor and condenser connected between said second switch and said connecting means, a second thermionic tube having a cathode, grid, screen grid and plate, said second thermionic tube serving as a reactance tube, a multi-tapped resistor connected between the anode circuits of said first and said second thermionic tubes, a third switch for selectively regulating the value of resistance of said multi-tapped resistor, a single means for simultaneously actuating said i'lrst, said second, and said third switches, apparatus to be tested, variable means to supply energy from said output circuit to said apparatus to be tested, a cathode ray tube having horizontal and vertical deflection means, means to supply energy from said apparatus to said vertical .deection means, a supply source of alternatin-g current, and a rectier connected between said supply source and said first and said secondv thermionic tubes, said supply source also supplying alternating current potentials to both the` control grid of said second thermionic tube and said horizontal deflection means.

AIFRED w. BARBER, 

